Flux density and VLBI measurements of the IDV source 0917+624 [Elektronische Ressource] / vorgelegt von Simone Bernhart
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Flux density and VLBI measurements of the IDV source 0917+624 [Elektronische Ressource] / vorgelegt von Simone Bernhart

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Flux Density and VLBI Measurementsof the IDV Source 0917+624DissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen Fakulta¨tderRheinischen Friedrich-Wilhelms-Universita¨t Bonnvorgelegt vonSimone BernhartausRuppichterothBonn 2010Angefertigt mit Genehmigung der Mathematisch-NaturwissenschaftlichenFakulta¨t der Rheinischen Friedrich-Wilhelms-Universita¨t Bonn1. Gutachter: Prof. Dr. Ulrich Klein2. Gutachter: Priv.-Doz. Dr. Walter HuchtmeierTag der Promotion: 15.03.2010iiA great pleasure in life is doing what people say you cannot do.Walter BagehotiiiContents1 Introduction 11.1 Active Galactic Nuclei and Relativistic Jets . . . . . . . . . . . . . . . . . . . . 11.1.1 Unified Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.2 Galactic Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Variability in AGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3 Objective and Structure of this Thesis . . . . . . . . . . . . . . . . . . . . . . . 72 Theoretical Basics 112.1 Compact Radio Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.1 Brightness Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.2 Superluminal Motion, Relativistic Beaming and Doppler Boosting . . . . 122.2 Intraday Variable Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.1 The IDV Phenomenon . . . . . . . .

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Published 01 January 2010
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Flux Density and VLBI Measurements
of the IDV Source 0917+624
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakulta¨t
der
Rheinischen Friedrich-Wilhelms-Universita¨t Bonn
vorgelegt von
Simone Bernhart
aus
Ruppichteroth
Bonn 2010Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen
Fakulta¨t der Rheinischen Friedrich-Wilhelms-Universita¨t Bonn
1. Gutachter: Prof. Dr. Ulrich Klein
2. Gutachter: Priv.-Doz. Dr. Walter Huchtmeier
Tag der Promotion: 15.03.2010
iiA great pleasure in life is doing what people say you cannot do.
Walter Bagehot
iiiContents
1 Introduction 1
1.1 Active Galactic Nuclei and Relativistic Jets . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Unified Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Galactic Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Variability in AGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Objective and Structure of this Thesis . . . . . . . . . . . . . . . . . . . . . . . 7
2 Theoretical Basics 11
2.1 Compact Radio Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Brightness Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.2 Superluminal Motion, Relativistic Beaming and Doppler Boosting . . . . 12
2.2 Intraday Variable Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1 The IDV Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.2 Intrinsic Explanations . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 Extrinsic Explanations . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Polarisation Properties of AGN . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1 Magnetic Fields and Synchrotron Radiation . . . . . . . . . . . . . . . . 20
2.3.2 Polarisation and Stokes Parameters . . . . . . . . . . . . . . . . . . . . 22
2.3.3 Faraday Rotation and Depolarisation . . . . . . . . . . . . . . . . . . . . 23
2.4 Precession of the Jet Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3 The Quasar 0917+624 27
4 Effelsberg Flux Density Monitoring of 0917+624 33
4.1 Observations and Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1.1 Data Reduction Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
v4.1.2 Tools for IDV Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5 The Kinematics of 0917+624 between 1999 and 2007 at Different Frequencies 47
5.1 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.2.1 Source Kinematics at 15 GHz . . . . . . . . . . . . . . . . . . . . . . . 54
5.2.2 Source Kinematics at 5 GHz . . . . . . . . . . . . . . . . . . . . . . . . 68
5.2.3 Source Kinematics at 22 GHz . . . . . . . . . . . . . . . . . . . . . . . 75
5.2.4 Combining All Frequencies - Spectral Evolution . . . . . . . . . . . . . 85
5.2.5 The binary black hole scenario for 0917+624 . . . . . . . . . . . . . . . 93
6 VLBI Polarimetry of 0917+624 97
6.1 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.2.1 Polarisation at 5 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.2.2 Polarisation at 15 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.2.3 Polarisation at 22 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.2.4 Rotation Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7 Discussion and Conclusions 125
8 Summary and Outlook 133
A Appendix A 139
B Appendix B 143
C Appendix C 153
C.1 The model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
C.2 The global method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Bibliography 165
D Acknowledgements 177
Acknowledgements 177
viC 1
Introduction
Black holes are among the most fascinating objects in the universe - in any case intriguing
enough for me so as to become an astronomer. It is today generally accepted that black holes are
located in the center of most galaxies. One particular species of galaxies is that of the so-called
Active Galactic Nuclei (AGN) which display energetic phenomena in their central region that
are comparable to or even exceed the energy emitted by all of the galaxies’ stars by orders of
magnitude. The first group of AGN with high central surface brightness, that was observed in
the optical in the early 1940s by Carl Seyfert, had not been considered significant until 1955
when the AGN were identified as radio sources. In the late 1950s the first radio surveys were
performed which enabled an identification of the strongest radio sources with their optical
counterparts which were usually galaxies, but sometimes appeared to be star-like objects. This
consequently led to the term quasi-stellar radio source or quasi-stellar object which later turned
into ’quasar’ or ’QSO’. Only the discovery of Schmidt (1963), that the optical spectrum of the
star-like object 3C273 is in fact highly redshifted and hence of cosmological origin, enabled an
efficient identification of quasars.
In order to put this thesis into a larger context, I shall first give an introduction to the
large variety of AGN and part of their characteristics on which this work is based.
1.1 Active Galactic Nuclei and Relativistic Jets
The radio structure of AGN can generally be referred to as either extended, i.e. spatially resolved,
or compact, i.e. unresolved at the corresponding observing frequency. In the radio bands, the
extended structure is usually double in nature showing two ’lobes’ on either side of the central
source. These structures can reach up to megaparsec dimensions. The lobes are connected to2 1. Introduction
the central region via relativistic jets, i.e. plasma streams which seem to transport particles and
energy to the outer lobes with relativistic speeds while emitting synchrotron radiation (e.g.,
Begelman et al. 1984, see also 2.3.1).
Up to the present, numerous work has been done on the structural properties of AGN and
their evolution. But very important questions, such as the detailed process of jet formation,
still remain unanswered. However, the knowledge about what is going on in the small central
regions of those active galaxies has improved considerably during the past decades. Today we
are familiar with a large variety of subclasses of AGN, one of which are Seyfert Galaxies,
named after their above mentioned discoverer. Seyfert Galaxies are lower-luminosity AGN
with a quasar-like nucleus, the host galaxy still being clearly detectable in the optical bands.
They can be divided into two subclasses: Seyfert type 1 galaxies show narrow emission lines
referring to low-density ionised gas as well as broad emission lines which can be attributed to
high-density gas. In Seyfert type 2 galaxies, only the narrow lines are present. In polarised
light, however, also the broad lines become visible. It is not jet fully understood what causes
the difference between Seyfert type 1 and type 2 galaxies. One hypothesis says that the two
types are intrinsically the same exhibiting both broad and narrow emission lines, however,
Seyfert 2s are probably observed from a different viewing angle such that the broad line region
is hidden by a circumnuclear torus. The radio luminosity of Seyfert Galaxies is only moderate
1compared to other more active AGN. Quasars , on the other hand, the most distant objects
in the universe, are most luminous at every wavelength at which they have been observed,
45 48 −1with bolometric luminosities in the range of 10 to more than 10 ergs s . They display
time-variable continuum flux and broad emission lines and often a large ultraviolet (UV) flux
specified as UV excess. Their high redshift indicates that quasars emerged in an early phase of
the universe which makes them an important cosmological probe. One distinguishes between
radio-loud and radio-quiet quasars, the latter originally referred to as QSOs (quasi-stellar
object). Today the terms ’quasar’ and QSO are virtually equivalent.
Radio Galaxies are usually found to be giant ellipticals showing million times brighter
radio luminosity than normal galaxies, although some of the brightest radio galaxies host in fact
quasar-like nuclei. The bulk radio emission is concentrated in the core and the afore mentioned
radio lobes. Fanaroff & Riley (1974) classified the radio galaxies according to their morphology
into FRI sources with weaker radio flux being brightest in the center, and FRII sources which
have well-defined jets and are more luminous and limb-brightened, i.e. they show hot spots
in their radio lobes. The transition specific luminosity between the two classes is defined
32 −1 −1as L (1.4GHz)≃ 10 erg s Hz (Bridle & Perley 1984). Furthermore, we can distinguishν
between broad-line radio galaxies (BLRG) and narrow-line radio galaxies (NLRG) as the
1The term ’quasar’ originates from their appearance as a quasi-stellar radio source.1.1. Active Galactic Nuclei and Relativistic Jets 3
radio-loud analogs to Seyfert galaxies of type 1 and 2.
The main target of this work, 0917+624, belongs to the class of Blazars, which subsume
2a group of radio-loud, core-dominated flat-spectrum radio sources consisting of BL Lac
objects, named after the prototype BL Lacertae, and the so-called optically violent variable
(OVV) quasars. They have in common that they are extremely variable on all time scales
and in all bands of the electromagnetic spectrum and tend to have high polarisation of up to
a few percent (in contrast to <1% for most other AGN). Besides, their jet is pointing almost
directly towards us enclosing only a small angle with the line of sight. Aside from that, BL Lac
objects do have no or only weak spectral line emission and absorption features compared to
the continuum, which often impedes a redshift detection (usually z ≤ 2), whereas OVVs have
broad emission lines as long as the continuum is not at its brightest (z ≥ 0.5). It has become
generally accepted that within the blazar class highly variable quasars are related to intrinsically
powerful FRII radio galaxies, and BL Lac objects are related to lower luminosity FRI galaxies.
This distinction explains the different emission line properties.
The above described types are the most common classes. Since they are partly divided
into subclasses according to their overall spectral energy distribution or emission line types, it
has been found a more general distinction between type 1 AGN (showing both broad and narrow
emission lines) and type 2 AGN (only narrow emission lines), and radio-quiet and radio-loud
objects. The term ’radio-loud’ refers to the ratio R of radio to optical emission at 6 cm and
4400 Å and is generally in the range 10–1000 for radio-loud objects (Kellermann et al. 1989).
1.1.1 Unified Scheme
One of the main achievements in AGN research is the idea about a unification of the various
different types of AGN, the characteristics of which I will shortly summarise in the following.
Since it is assumed that the above described properties of AGN are mainly depending on the
orientation towards the observer and are less due to real physical differences like, e.g., the
luminosity, it was attempted to find a kind of morphological model which is able to describe
AGN with as few parameters as possible (see, e.g., Urry 2004, Peterson 1997).
The black-hole paradigm states that AGN host a ”central engine” consisting of a supermassive
7 10black hole (10 -10 M ) encircled by an accretion disk, in which gravitational potential energy⊙
is converted into radiation (ranging from radio to X-ray). Surrounding this is the broad-line
region (BLR), consisting of high velocity gas, followed by the lower density and lower velocity
gas of the narrow-line region (NLR). Around the unresolved components of an AGN there
lies an optically thick obscuring torus that permits the AGN radiation to escape only along the
2 αSource flux density S depends on observing frequency via S ∝ν with power-law indexα≥−0.5ν4 1. Introduction
Figure 1.1: Schematic of an AGN according to Urry & Padovani (1995); according to the orientation, this
would be a radio-loud AGN.
torus axis, which leads to observable large-scale ionisation cones. Relativistic jets, formed
within.100 Schwarzschild radii of the black hole (most likely due to the existence of a toroidal
magnetic field), extend outwards along this torus axis for tens of kpc up to Mpc in some
cases (see Figure 1.1 for a schematic illustration). The alignment of the torus/jet axes seems
to be independent from the rotation axis of the host galaxy (see, e.g., Schmitt et al. 2002 and
references therein).
Apart from intrinsic variations in the black hole mass, size, density, luminosity, etc., the
above basic description is apparently valid for all AGN - with an exception being the relativistic
jets. These are found in most radio-loud AGN but with differences concerning their kinetic
powers. In contrast to the powerful radio jets that expand far into the intergalactic medium,
weaker jets deccelerate relatively close to the central engine, which can be even within the host
galaxy. It is not yet well understood why only 5-10% of the AGN are radio-loud.
Local obscuration has been studied by Antonucci & Miller (1985), who found that the
distinction between type 1 and type 2 AGN seems mainly to be due to the orientation with